This invention relates to a method of producing a hermetically sealed ceramic to metal bond for implantation in living tissue.
Known methods of bonding a ceramic to a metal involve the use of interlayer materials which either melt at the bonding temperature, such as a braze, or which involve special coating processes for the material surfaces to be bonded, such as pre-coating the surfaces with an activating material. In some methods of bonding, an interlayer material having a composition that approximates the composition of the initial metal bonding surface is utilized, such as disclosed by Lasater (U.S. Pat. No. 6,221,513 B1). Lasater describes a method for forming a hermetically sealed bond for use in implantable medical devices. Hill (U.S. Pat. No. 3,594,895) described another approach to forming a ceramic to metal seal.
Cusano (U.S. Pat. No. 3,994,430) disclosed a method of directly bonding metal to ceramic substrates wherein a very thin layer of an interlayer material is placed between the metal and the ceramic to be bonded. The system is heated in an inert atmosphere to a temperature between the eutectic temperature of the interlayer material and the melting point of the metal. Cusano focused on bonding copper to a ceramic substrate, such as alumina or beryllia.
The present invention relates in general to the art of diffusion bonding in a non-reactive atmosphere, a flat ceramic rod end to an approximately matching flat metal rod end, particularly an yttria-stabilized zirconia rod to a titanium alloy rod, including the alloy Ti-6 Al-4 V, in order to produce a hermetic seal between the metal and the ceramic for use in living tissue, especially an animal body.
A very thin, approximately 0.001 inch thick or less, interlayer material of specific composition, typically essentially pure nickel or a pure nickel alloy, is placed between the surfaces of the parts to be bonded, the parts are yttria-stabilized zirconia and a titanium alloy. The assembly, with the surfaces held together at stress sufficient to cause intimate contact between the parts, is heated to the bonding temperature, between approximately 1728xc2x0 and 1800xc2x0 F., where the interlayer material and the metal surface exchange atoms in a solid state diffusion process involving little or no volume change. The joint isothermally solidifies in a short period of time, on the order of approximately 5 minutes, depending on the exact temperature, bonding stress, and configuration. The initial bond strengthens with additional time at temperature or with subsequent heat treatment, and is fully developed in approximately 15 minutes or less at temperature.
The bonding temperature is less than the melting point of any of the materials being bonded and is approximately equal to or slightly greater than the temperature of the eutectic formed between the interlayer material and the metal part.
This invention eliminates problems of the prior inventions. For example, because there is no melting of the interlayer material, there is little or no flow of the interlayer material out of the joint as xe2x80x9cflashxe2x80x9d. In known methods, the flash must be removed in a post-bonding process. Further, the flash often bonds to the furnace hardware during thermal processing, making removal of the finished part difficult. This invention relies on solid state bonding and diffusion and does not involve melting of the interlayer material per se, thus allowing the original pre-bond dimensions to be preserved during the bonding process.
The bonding process takes place in a non-reactive atmosphere, such as in a vacuum or in an inert gas, such as argon, while the metal and ceramic parts being bonded are held together with a pressure that is sufficient to maintain intimate contact of the parts.
The resulting component assembly that comprises the metal, ceramic, and the metal-to-ceramic bond is biocompatible with living tissue when implanted in an animal body. The component assembly resists electrolytic corrosion and crevice corrosion.
The bonded component assembly may be subjected to an acid etch to eliminate any free nickel or nickel salts that remain a part of the component assembly, although the nickel is generally completely tied up as an integral part of the completed component assembly. Nickel and nickel salts are harmful to living tissue.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
It is an object of the invention to provide a component assembly having a strong bond between a ceramic and a metal part that is biocompatible and resists electrolytic corrosion when implanted in living tissue.
It is an object of the invention to provide a strong bond between a ceramic, such as yttria-stabilized zirconia, and metal, such as a titanium alloy.
It is an object of the invention to provide a hermetic seal between a ceramic, such as yttria-stabilized zirconia, and metal, such as a titanium alloy.
It is an object of the invention to provide a method of bonding a ceramic to a metal for implantation in living tissue.
It is an object of the invention to provide a method of bonding a ceramic to a metal where there is no dimensional change in the joint during thermal development of the bond.
It is an object of the invention to provide a method of bonding a ceramic to a metal where there is no flow of the interlayer bonding material from the joint during thermal processing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.